Process for solution mining kci deposits



Jan. 30, 1968 l. s. PASTERNAK ET AL. 3,366,419

PROCESS FOR SOLUTION MINING KCL DEPOSITS Filed Nov. 12, 1964 4sheets-Sheet l x X XX X X x x x f Israel S. Pasfernak John L-TiedieINVENTORS.

TTORNEY Jan. 30, 1968 s. PAsTERNAK ET AL 3,366,419

PROCESS FOR SOLUTISN MINING KCL DEPOSITS 4 sheets-Sheet Filed Nov. 12

Israel S. Pasfernak John L. Tiedje INVENORSL BY M TTORNEY Jan. 30,1968 1. s. PASTERNAK ET AL 3,366,419

PROCESS FOR SOLUTION MINING KCL DEPOSITS Filed Nov. 12, 1964 4sheets-Sheet 4 BRINE FROM NON-SELECTIVE SOLUTION MINING CONTINlNG NclClKCI IN FORMTION RTIO AND SATURATED WITH NaCl -DWTER RECYCLE T0 CAVITIES/3| oPERATmG NoN-sELEcTlvELv 35\ MULTIPLE EFFECT /33 EVAPORATORSCRYSTLLIZERS NGC| SOLUTION RECYCLE T0 CVITIES OPERTING SELECTIVELY BRINEFROM SELECTlVE SOLUTION MlNlNG WITH NGCI SOLUTION Israel S. PasfernakJohn L. Tfedje INVENTORS BY M v v ATTORNEY United States Patent O3,366,419 PROCESS FOR SOLUTION MINING KCl DEPOSITS Israel S. Pasternakand John L. Tiedje, Sarnia, Ontario, Canada, assignors to Esso Researchand Engineering Company, a Corporation of Delaware Filed Nov. 12, 1964,Ser. No. 410,411 12 Claims. (Cl. 299-5) ABSTRACT OF THE DISCLOSURE Thisdisclosure relates to methods of solution mining of soluble saltdeposits with a suitable solvent and is characterized by a particularmethod of well formation and by alternating cycles of selective andnonselective dissolution of the salt.

This invention relates to the mining of soluble minerals fromunderground strata and, particularly, to the solution mining of asoluble salt. More particularly, the invention is concerned with therecovery of potassium chloride from sylvinite by a particular method ofsolution mining. Most particularly, the invention is concerned with therecovery of potassium chloride from sylvinite by a method of solutionmining characterized by a particular method of well formation and byalternating cycles of selective and nonselective dissolution of thesalt.

Sylvim'te is a physical mixture of sylvite (KCl) and halite (NaCl) whichoccurs in natural deposits in various places throughout the world. Theratio of KCl to NaCl in the sylvinite deposits varies widely. Not onlydoes the ratio of sylvite to halite vary from one geographic location toanother, but the ratio of the two salts may vary at diierent depths inthe same deposit.

A sylvinite deposit sufliciently rich in KCl can be selectively solutionmined with a saturated sodium chloride brine. The dissolved potassiumchloride may be recovered at the surface simply by cooling the saturatedbrine, collecting the precipitated potassium chloride crystals andrecycling the -supernatant brine, With or without the addition of makeupwater. However, in the case of KCl-lean ores, once the surface crystalsof potassium chloride are dissolved, no further potassium chloride isexposed since the sodium chloride crystals do not drop away from thesolution face. Thus, the lean deposits require nonselective mining Withwater, or with unsaturated brine, whereby the salt formation isdissolved completely producing solutions which are usually saturatedwith sodium chloride at cavity temperature and which contain potassiumand sodium chlorides dissolved in the naturally occurring ratio of theformation.

Heretofore in the art, various methods have been used to mine sylvinite.The usual method for mining sylvinite in order to obtain KCl was todrill from the surface down into the bed of the sylvinite and theninject water into the drilled hole, dissolving the sylvinite, returningit to the surface, and separating the KCl from the NaCl. This procedurerequired the use of extensive separating means at the surface and addedto the cost of the KCl. Another method is that set forth in U.S. PatentNo. 3,096,969 for the mining of sylvinite ore which contains KCl in aconcentration of at least 15 wt. percent. This method comprised drillingfrom the surface into the bed of the sylvinite down to a layer which wasrich in NaCl, utilizing controlled mining in order to remove thesylvinite laterally from the bottom of the drilled well and 3,355,419Patented Jan. 30, 1968 ICC then mining the KCl-rich layer containingmore than 15 wt. percent of KCl by filling the lateral excavation withwater (or an aqueous solution -dilute in NaCl and KCl) and then miningthe KCl-rich layer by gradually allowing the water to rise verticallyinto the KCl-rich layer.

It is the object of the present invention to provide a process ofsolution mining which comprises a particular method of mining a solublesalt.

It is the object of the present invention to provide an improved methodof roof control of the cavity in solus tion mining by forming a blanketof oil above the solution and by correlating the blanket thickness withthe diameter of the cavity as it is being formed.

It is an object of the present invention to provide a process ofsolution mining which is especially suited for the exploitation of asylvinite deposit wherein the ratio of sylvite to halite variesconsiderably. It is a further object of the invention to provide aprocess of solution mining for the efiicient recovery of potassiumchloride from sylvinite deposits wherein the selective dissolution ofpotassium chloride is only partially feasible.

A great many sylvinite deposits are not rich enough in KCl to beselectively mined completely. However, some of these deposits containrich bands of KCl which can be mined selectively. The present inventionis based in part upon the discovery that, in general, sylvinite orescontaining less than about 35 sylvite cannot be selectively mined sincethe removal of potassium chloride does not cause the remaining sodiumchloride crystals to drop away from the solution face, whereas depositscontaining about 35 or more of KCl can be selectively solution minedbecause the remaining sodium chloride crystals drop away freely from thesolution face, thereby continuously exposing additional potassiumchloride. In certain deposits of sylvinite ore, the criticalconcentration of KCl required for selective mining will deviate frombetween about 30 to 40%. For example, ores which contain more than theusual amount of insoluble impurities are more amenable to selectivemining and may require a KCl content of only 30 to 35%, whereas certainother deposits Vcontaining less impurities are not as amenable toselective mining and may require a KCI content of 36 to 40%. The exactpercentage required for the selective mining of a given ore is readilydeterminable, as will appear from later discussion. The usual percentageof KCl in sylvinite which atfords the selective mining of KCl inaccordance with the instant invention is about 35% based on the totalamount of KCl and NaCl in the sylvinite. However, the selective miningof this invention can be carried out when the weight percent of KCl isas low as 30%.

In accordance with one embodiment of the present nvention, nonselectivemining is carried out by the continuous injection of water during thecontact of sylvinite ore containing less than about 30 to 40% KCl,followed by periods of selective solvation of an ore containing KCl inconcentrations of 35% or more carried out by the injection of sodiumchloride brine. During the latter cycles, crystals of potassium chloridewill be selectively dissolved from the ore face leaving the sodiumchloride Icrystals to fall to the bottom of the cavity together withsome displaced sodium chloride Originally present in the injected brine.When the rich ore band containing more than 30 to 40% KCl has beenremoved, as determined by analysis of brine sampled from near the top ofthe cavity, nonselective solution minning using water injec- 3 tion isagain resumed until another rich KCl band is encountered.

In accordance with a further embodiment of the invention, nonselectivemining is also carried out initially -by the continuous injection ofwater during the contact of sylvinite ore containing less than about 30to 40% KCl. When analysis of the sampled brine indicates that a stratumcontaining at least about 30% KCl by Weight has been encountered, thewater injection is converted from continuous to semicontinuous flow.Between the periods of water injection, the cavity is shut in to allowsuccessive batches of water to become saturated with both sodium andpotassium chloride.

Each batch of water first becomes saturated with sodium chloride andalso dissolves an amount of potassium chloride corresponding to thenaturally occurring ratio of the formation. Then, -because of the highconcentration of potassium chloride in the ore, crystals of potassiumchloride are selectively dissolved. When this occurs sodium chloridecrystals drop away from the face of the ore formation and fall to thebottom of the cavity. At the same time, some sodium chloride will bedisplaced from solution by potassium chloride and also fall to thebottom of the cavity. The KCl content of the produced solution is thusincreased. The sodium chloride crystals falling to the bottom of thecavity remain in a layer of saturated solution throughout the life ofthe cavity and are not redissolved.

A more complete understanding of the invention may be obtained from theaccompanying drawings.

FIGURE 1 illustrates the preferred well completion assembly forpracticing the method of the invention. It also shows the conditionswhich exist in the salt deposit after the initial stages of the processare carried out.

FIGURE 2 illustrates the geometry of the cavity created during theintermediate stages of the process.

FIGURE 3 illustrates the cavity profiles characteristic of latter stagesof the process.

FIGURE 4 is a schematic diagram of a surface processing system for usein connection with alternating cycles of nonselective mining with waterand selective mining with sodiu-m chloride brine. It is also useful inconnection with intermittent water injection.

Referring now to FIGURE l, borehole 11 is drilled from the surface ofthe earth through the various layers of the salt deposit to the bottomof zone 12, the lowermost KCl-containing stratum to be mined. Casing 13is set in the hole to the uppermost level of the salt deposit. Threetubing strings are then lowered into the casing.

The lower end of tubing 14 is for brine removal and is inserted to alevel near the lower Vboundary of the potash containing ore. The lowerend of tubing 15 is for water njection and is positioned about to feetabove the brine removal tubing. Tubing 16 is for blanket level control(gas or oil) and is inserted to a level about 1 foot higher than thelevel of water injection tubing 15.

Cylindrical chimney 17, for example, about 10 to 20 feet in diameter isthen washed out between the bottom and the top of the potash formationby simply passing water down tubing at a rate of about 500 to 5,000

barrels per day and removing the produced brine by way of tubing 14.When the top of chimney 17 reaches the uppermost level of thepotash-containing formation, as indicated by conventional loggingtechniques, as well as by a reduced rate of KCl production, the chimneyis filled with oil or gas, which is injected into the casing by way oftubing 18 and is maintained at a level about one foot above the end ofthe water injection tubing.

The development of chimney 17 during the initial stages of the operationallows a relatively large vertical area for dissolution of the potash atthe beginning of the undercutting stage of the overall operation. Italso serves as a reservoir for the blanket control fluid (oil or gas)supplied from the surface, ensuriag an adequate oil supply at a desiredlevel during the subsequent mining stages of the operation. Thus, duringthe latter stages of the operation, little or no blanket fluid need besupplied from the surface.

With the blanket fluid in place, the Washing out of an undercut is begunas shown in FIGURE 2. This stage involves the continued injection ofwater, and the removal of the resulting potash solution from the cavityWhile maintaining the level of the oil blanket constant by the injectionof oil around the casing and the removal of excess oil from the blanketlevel control tubing. In many cases, the use of the blanket levelcontrol tubing may be omitted and oil added at a rate suflicient tomaintain an oil lblanket thickness of up to two inches duringundercutting. However, such a method of Controlling the oil blanketthickness has certain disadvantages as the diameter of the undercut isincreased. According to the present invention, the blanket level controltubing 16 is lowered at least about one inch and preferably from 1 to 2inches for every 50 feet of increasing cavity diameter during theWashing out of the undercut. Concurrently with each lowering of theblanket level control tube, an additional amount of blanket fluid isintroduced through the casing sufficient to insure an underflow ofblanket fluid into level control tubing 16. Solution of the potassiumchloride occurs at the circumference of the cavity below the oil blanketand above the brine outlet tube. This causes undercut 19 to grow in ahorizontal direction, its shape being approximately that of a circulardisc 5 to 15 feet in height, depending upon the distance between the oilblanket and bottom of the brine removal tube 14. The undercutting iscarried on until the diameter becomes greater than 300 feet at whichtime the oil blanket is raised about 5 to 15 feet by lifting the oilcontrol tubing. In normal operation, the preferred height of thecircular disc and also the distance the oil control tubing is raised isabout 10 feet. However, if the particular layer of sylvinite, that is,either the KCl or the NaCl-rich layer, is either more or less than 10feet thick, then the oil control tubing is moved a correspondingdistance thereby providing for cutting substantially within the KCl orNaCl layer.

FIGURE 2 shows a sketch of the cavity configuration at the end of theundercutting period. In order to maintain an undercut of fairly uniformdepth, it is important that a high flow rate (at least 4,000 to 6,000barrels per day, preferably 5,000 barrels per day) be used to preventthe produced brine from becoming saturated. If this is not done, analmost saturated solution will be produced causing the floor of theundercut to rise as the diameter is increased.

After the undercutting stage of the operation, the protective blanket israised, about 10 feet for example, thereby exposing an extremely largehorizontal area for dissolution of the potassium chloride.

FIGURE 3 illustrates successive changes in the crosssection of thecavity during the mining of layer 20 and of subsequent stages of miningin accordance with the alternating cycles of the invention. To preventdilution of the exit brine lwith water injected at high rates, the waterinjection tube 15 is raised such that its outlet end is just below theoil blanket. The separation between the bottom of the injection tubingand the bottom of the brine or water removal tubing is thereby kept at amaximum. The oil level is raised in successive stages of about 10 feet,for example, during which the cycling of nonselective and selectivesolution mining is continued until a cavity having a heightcorresponding to the entire salt formation is obtained having a diameterof several hundred feet.

Here it should be noted that periodic samples of brine should beobtained from near the top of a cavity and analyzed for NaCl and KCl inorder to determine when to change from a nonselective to a selectiveperiod of solution mining or vice versa. This can readily beaccomplished by flowing a small quantity of brine up through the oillevel control tube in between periods of oil injection into the cavity.This procedure is necessary to eliminate the long residence time of theproduced brine in a large diameter cavity.

FIGURE 4 is a schematic diagram of the surface processing operation usedin connection with the alternating nonselective and selective cyclesdescribed above. Stream 31 containing brine from a nonselective cycle ofthe mining process is passed together with recycle stream 32 into abattery of conventional multiple effect evaporators 33. Feed stream 31contains produced brine in which sodium and potassium chloride aredissolved in the naturally occurring formation ratio. The brine issaturated with respect to the sodium chloride. In the multiple effectevaporators 33, sodium chloride crystals are selectively precipitatedfrom the brine and removed at 34. During this process, evaporated wateris recovered and recycled through line 35 to the salt cavity on acontinuous or semicontinuous basis, depending upon the stage of theprocess being carried out as described above.

A brine solution saturated with respect to both potassium and sodiumchlorides is passed from zone 33 through line 36 to conventionalcrystallization zone 37. The produced brine from a cycle of selectivemining is passed by way of line 38 to crystallizers 37 along with thesaturated brine from evaporators 33. The brine of line 38 is thatproduced from either of the abovedescribed embodiments of the invention.That is, it may be produced from a cycle of continuous sodium chloridebrine injection, or it may be produced after a shut-in period ofsemicontinuous water injection. In crystallization zone 37, the combinedbrines are cooled whereby potassium chloride is selectively precipitatedand removed through line 39. The resulting solution is recycled in partthrough line 32 to evaporators 33. The remainder of the recycle streamis returned by way of line 40 to one or more cavities being operated ona cycle of selective mining by injection of a saturated sodium chloridebrine.

As described earlier, the brine samples obtained periodically from oillevel control tubing are analyzed in order to detect the exposure of aKCl-rich stratum in the cavity or cavities being nonselectively mined.Since these brine samples contain sodium and potassium chloride in thenaturally occurring formation ratio, the analysis ratio NaCl/KCl gives adirect indication of ore quality currently being mined. Thus, ingeneral, when the analysis ratio falls to about 2.4, preferably 1.9, orbelow, the nonselective cycle is interrupted and a selective cycle isbegun. Conversely, a selective cycle is interrupted when the analysisshows that the solution is becoming unsaturated With respect to KCl,`and nonselective mining is resumed.

A critical ratio of 2.4 to 1.'9 Will not always be satisfactory in themiuing of certain ores. As mentioned earlier7 some deposits are moreamenable to selective mining than others. Thus, at the beginning of aselective cycle, if the KCl content of the sampled brine fails toapproach saturation, nonselective mining is resumed until the NaCl/KClratio of the ore falls to 1.8, for example, at which time selectivemining is again attempted. If the mining solution is not saturated atthis NaCl/KCl ratio, nonselective mining is again resumed untilselective mining can be conducted. In this manner, a satisfactory ratioof from 2.4 to 1.6 is readily determined for a given ore.

With the beginning of each new stage of the operation, the blanket levelcontrol tubing 16 is raised about feet, for example, as mentioned above.After the lower level of the blanket control fluid is raised for thepurpose of beginning a new stage of the process, improved roof controlis obtained, as before, by the step of lowering the end of blanket levelcontrol tubing 16 at least about 1 inch for every 50 feet of increasingblanket diameter during the Washing out of layer and each successivelayer of the salt deposit. Concurrently with each lowering of theblanket level control tube, an additional amount of blanket fluid isintroduced through the casing, sufiicient to ensure a continuousunderflow of blanket fluid into level control tubing 16. In this way acontinuous driving force is obtained to promote oil (or gas) movement ina horizontal plane, thereby reducing the danger of vertical solventbreakthrough into the cavity roof.

While the present method of mining potassium chloride has been describedwith particular reference to a single-Well operation, it will be readilyappreciated that a multiplicity of boreholes or wells may also beutilized. For example, the process may be carried out by sinking twoboreholes to the base of the salt deposit, establishing communicationbetween the holes, injecting unsaturated solution through one well andwithdrawing saturated solution from the other well.

In connection with such a two-hole system of operation, seriousdificulties sometimes arise. The removal of saturated brine through theIproduction tubing string frequently results in a substantial cooling ofthe brine. Crystallized potassium chloride is thereby caused toprecipitate in suflicient quantities to cause plugging of the productiontubing.

In accordance With a further embodiment of the invention, this problemis solved by installing a second tubing string in the same hole with thebrine production tubing and passing heated water or steam therethroughat a rate which is just sufiicient to keep the produced brine at orabove cavity temperature, thus preventing crystallization. The bulk ofthe solvent water is, of course, still injected into the cavity throughthe other hole. During the mining operation, the water injected throughboth the holes Will dissolve potassium chloride ore, become saturated atcavity temperature and be removed through the brine production tubing.The stream used to heat the produced brine must be injected into thecavity at a level which is high enough to prevent the production of anunsaturated brine by means of dilution in t-he vicinity of theproduction borehole.

There are two primary advantages for circulating hot water or steam inthe production borehole as above described. While it prevents thecrystallization of potassium chloride in the production tubing, itnevertheless allows a large diameter tubing to be used for brineproduction, because only a small diameter line is necessary for theintroduction of a sufiicient quantity of hot water or steam to maintainthe necessary temperature for the prevention of crystal formation. Thiscauses little or no increase in `the horsepower requirements for pumpingthe total quantity of water into the cavity. Since the brine producedfrom the cavity during nonselective solutioning must be heated before itis processed in the surface plant, the heat added to the water to keepthe produced brine hot may be considered useful as a preheat step forsurface processing. Thus, the additional cost of employing the hot wateror steam injection is small and the heat exchange required in surfaceprocessing may accordingly be reduced in size.

During the nonselective mining period of the process, large quantitiesof sodium chloride are produced, for Which no adequate market usuallyexists. In accordance with a further embodiment of the invention, thesurplus sodium chloride is returned to the cavity during periods ofselective solution mining. A return of sodium chloride to the cavityamounts to more than a mere disposal or surplus by-product. Sodiumchloride deposited in the solution cavity displaces a brine rich inpotassium chloride which otherwise Would remain trapped in the abandonedcavity. Moreover, the filling of the cavity with solid sodium chlorideprovides a significant measure of support for lthe cavity roof whichcontributes to the prevention of serious subsidence at the surface.

To obtain a better distribution of suspended sodium chloride throughoutthe cavity, it is frequently desirable to provide brine slurry injectionand brine Withdrawal through separate wells laterally spaced somedistance apart. This situation would normally exist where a solutioncavity has been formed by circulation through two Wells. However,optimum distribution of the surplus salt in the cavity would be obtainedwhen injection and withdrawal Wells for sodium chloride disposal aredrilled at the extremities of an abandoned cavity.

In the case of reinjecting surplus sodium chloride during periods ofselective solution mining, the injected brine slurry is saturated withrespect to sodium chloride only and would, therefore, have a densitysomewhat lower than a produced brine saturated with both potassium andsodium chlorides. Because of this difference in density, conductiveforces would tend to promote a good distribution of settled sodiumchloride over substantially the entire cavity fioor. In this case, brinewithdrawal must be from a level near the bottom of the cavity in orderto iusure saturation with respect to vboth chlorides.

What isfclaimed is:

1. A method of recovering a soluble salt from an underground depositwhich comprises the steps of:

(a) drilling a borehole from the surface of the earth to the lowestboundary level of said deposit,

('b) establishing fluid communication between the surface of the earthand at least four separate levels within said deposit through saidborehole,

(c) injecting a solvent for said salt at the second lowest level offluid communication and removing the produced salt solution at thelowennost level of fluid communication,

(d) establishngla blanket reservoir of solvent-immiscible fluid from theuppermost level of fluid communication to the next lower level of fluidcommunication,

(e) continuing the injection of solvent at the second lowest level offluid communication within said deposit,

V(f) raising the lower level of said blanket reservoir in stages therebyrecovering salt from the next higher level of the deposit,

(g) continuing the removal of produced solution from the lowermost levelof fluid communication, and

(h) increasing the blanket thickness as a cavity is formed by dissolvingthe deposit laterally.

2. A method as in claim 1 wherein said blanket fluid is a hydrocarbonoil.

3. A method as in claim 1 wherein the blanket reservoir is raised insuccessive stages of about ten feet.

4. A method as in claim 1 wherein the blanket thickness is increased atleast about one inch for every fifty feet of increased cavity diameter.

5. A method as defined by claim 1 wherein the solvent for said salt is asaturated sodium chloride solution When said salt is sylvinitecontaining at least 30% KCl.

6. A method as in claim 5 wherein the sodium chloride from the sylvinitedeposit is returned to the cavity;

7. A method of recovering potassium chloride from a subterraneanWater-soluble salt deposit containing KClrich layers and KCl-leanlayers, which comprises the steps of drilling a borehole from thesurface of the earth through said salt deposit to the bottom of aselected KClcontaining stratum, establishing fluid communication betweenthe surface of the earth and at least four separate levels within saidsalt deposit through said borehole, injecting water at the second lowestlevel of fluid communication and removingproduced brine at the lowermostlevel of fluid communication, thereby forming a generally cylindricalcavity within said deposit, establishing a reservoir of awater-imrniscible blanket fluid from the uppermost level of fluidcommunication to the next lower level of fluid communication, thereaftercontinuing the injection of water at the second lowest level of fluidcommunication withsaid deposit, thereby establishing a disc-shapedundercut at the base of said salt deposit, thereafter raiSing the lowerlevel of said blanket reservoir in successive stages while continuingthe injection Vof water at said second lowest level of fluidcommunication within said salt deposit, continuing the removal ofproduced brine from the lowermost level of fluid communicationestablished within said Well lbore, rand alternating the injection ofWater within the injection of ya saturated sodium chloride brinewhenever the analysis of the brine sampled from near the top of thecavity ndicates that a band of potassium chloride-rich ore has beenencountered containing at least about 35 KCl;

8. In the solution mining of potassium chloride 'from subterraneanwater-soluble salt deposits comprising layers which contain more than35% KCl, alternating with layers which contain less than 35 KCl, theimprovement Which comprises the steps of nonselectively mining thoselayers which contain less than 35 KCl by the injection of an aqueoussolvent, analyzing the brine samples obtained from near the top ofthecavity and, when said analyses iudicates the exposure of a layerwithin said deposit containng more than about 35% KCl, converting fromnonselective mining to selective mining of potassium chloride byinjecting a saturated sodium chloride brine containing surplus sodiumchloride slurried therewith.

9. In the solution mining of potassium chloride from subterranean saltdeposits comprising layers which contain more than 35% KCl, alternatingwith layers which contain less than 35% KCl, the improvement whichcomprises the steps of injecting a saturated sodium chloride brine incontact With the potassium chloride-rich layers, said sodium chloridebrine contaning surplus sodium chloride slurried therewith, andalternately injecting unsaturated sodium chloride 'brine or water incontact with layers containing less than about 35% potassium chloride.

10. In the solution mining of potassium chloride from subterranean saltdeposits comprising layers which contain more than 35 KCl, alternatingwith layers which contain less than 35 KCl, wherein communication withsaid salt deposit is established through au input well and an outputwell laterally spaced therefrom, the improvement which comprisesinjecting a slurry of surplus sodium chloride contained in a saturatedaqueous solution of sodium chloride through said input Well in contactwith the potassium chloride-rich layers alternately, injectingunsaturated sodium chloride brine or water into contact with layerscontaining less than about 35% potassium chloride, removin-g a resultantbrine through said output well, and passing a small amount of hot wateror steam in indirect heat exchange with the stream of produced brine asit is being removed through said output well bore.

11. In the solution mining of potassium chloride from subterranean saltdeposits comprising sodium chloride and potassium chloride andcomprising layers which contain more than 35% KCl, alternating withlayers which contain less than 35% KCl, the improvement Which comprisesthe steps of nonselectively mining those layers Which contain less than35 KCl by the continuous injection of an aqueous solvent, analyzing thebrine samples obtained near the top of the cavity and, when saidanalyses indicate the exposure of layers within said deposit containingmore than about 35 KCl, converting from nonselective mining to selectivemining of potassium chloride by using an intermittant flow of theaqueous solvent, whereby the time between each successive flow ofsolvent is suficient to enable said solvent to become saturated withsodium chloride aud potassium chloride.

12. In the solution mining of potassium chloride from subterranean saltdeposits comprising sodium chloride and potassium chloride andcomprising layers which contain more than 35% KCl, alternating withlayers which contain less than 35 KCl, the improvement which comprisesthe steps of non-selectively mining those layers that contain less than35% KCl by the continuous injection of an aqueous solvent, analyzing thebrine samples ob- 9 10 tained near the top of the cavity and, when saidanalysis References Cited indeates NaCl/KCI ratio of at least 2.4,converting from UNITED STATES PATENTS non-selective minng to selectivemning of potassium 2665124 1/1954 Cross 299 5 Chloride using anifltermittent flow of the aqueous Brandt solvent, whereby the timebetween each suceessive flow 5 31096969 7/1963 Edmond's'gt' 299 4 ofsolvent is suficient to enable said solvent to become saturated withsodium chloride and potassium chloride. ERNEST R. PURSER, PrimaryExamner.

